Water is key to plate tectonics on Earth, which, in turn, is vital to the production of continental crust. Although arc lavas erupt in a volatile‐rich state and calc‐alkaline arc plutons are distinguished by the presence of hydrous minerals such as hornblende and biotite, the water content of arc magmas earlier in their evolution—in the deep crust—remains poorly constrained. Here, we report H₂O contents in nominally anhydrous minerals measured in situ on petrographic thin sections by secondary ion mass spectrometry of Proterozoic deep crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and garnet contain average H₂O contents ranging from 75–760, 233–410, and 42–139 ppm, respectively. Reconstructed bulk rock H₂O contents range from ~60 to ~650 ppm. Intermineral H₂O ratios overlap experimental mineral/melt D values and are used to calculate H₂O of melts last in equilibrium with the xenoliths. We propose that these xenoliths represent cumulates fractionated from a primitive, hydrous (≥1 wt.% H₂O) melt at high (~1 GPa) pressures, similar to conditions in modern subduction zones and potentially associated with widespread arc accretion that formed the core of North America in the Precambrian.

中文翻译：

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从石头中挤压水：粒状异岩和深水分数维结晶中名义上无水矿物中的H2O

水是地球板块构造的关键，而地球构造又对大陆壳的产生至关重要。尽管弧熔岩以挥发性丰富的状态喷发，并且钙碱性弧岩岩层以角闪石和黑云母等含水矿物的存在而著称，但弧岩浆在其演化的早期（在深地壳中）的含水量仍然受到限制。在这里，我们报道了通过来自美国科罗拉多州的元古代深地壳异种岩的二次离子质谱法在岩石薄片上原位测量的名义上无水矿物中的H ₂ O含量。Clinopyroxene，Orthopyroxene和石榴石的平均H ₂ O含量分别为75-760、233-410和42-139 ppm。重建的散装岩石H ₂O含量为约60至约650ppm。Intermineralħ ₂ O比重叠实验矿物/熔体d值和用于计算ħ ₂ ö最后与包体平衡熔体。我们建议这些异种岩代表在高压力（〜1 GPa）下从原始的含水（≥1 wt。％H ₂ O）熔体中分馏出来的堆积物，类似于现代俯冲带中的条件，并可能与形成熔体的广泛电弧积聚有关。北美前寒武纪的核心。